1. Field of Invention
The present invention relates generally to lithography. More particularly, the present invention relates to a stage for use in an electron beam projection lithography system.
2. Description of the Related Art
Lithography processes, e.g., photo-lithography processes, are integral to the fabrication of wafers and, hence, semiconductor chips. Conventional systems used for lithography include optical lithography systems and electron beam projection systems. Many optical lithography systems and electron beam projection systems may use a direct writing process to xe2x80x9cwritexe2x80x9d on wafers. However, direct writing processes are often relatively slow, as will be appreciated by those skilled in the art.
In order to increase the speed at which wafers may be written to, electron beam projection systems, as well as optical lithography systems, may project beams of finite area through patterns. The patterns are generally resident on a reticle, which effectively serves as a mask or a negative for a wafer. For an electron beam projection system, a relatively broad beam of electrons may be collimated and provided to a reticle, which may be a silicon wafer, e.g., a wafer that is suitable for scattering with angular limitation projection electron beam lithography or a stencil-type wafer. Typically, rather than absorbing the beam, the pattern deflects portions of the beam in order to prevent electrons from being ultimately focused onto a wafer.
FIG. 1a is a diagrammatic representation of one standard configuration of a lens system of an electron beam projection system. In general, a lens system 10 includes an illumination lens 12 and a projection lens 18. An electron beam is generated by an electron gas, and arranged to pass through illumination lens 12, through a reticle 16 held in a reticle chamber 14. It should be appreciated that reticle chamber 14 is typically a vacuum chamber. As an electron beam passes through reticle 16, portions of the electron beam are allowed to pass through reticle 16, while other portions of the electron beam may be scattered to prevent those portions from being focused onto a wafer 22 held in a wafer chamber 20, i.e., a vacuum chamber. In other words, reticle 16 acts as a mask to effectively mask out part of an electron beam. Projection lens 18 is arranged to project the pattern of electrons onto wafer 22.
In general, wafer 22 is mounted on a wafer stage (not shown) to facilitate the movement of wafer 22 beneath projection lens 18. The design of a wafer stage for use in an electron beam projection system may be complicated, as an electron beam projection system generally must not include moving magnets or metals which alter the magnetic field associated with the electron beam projection system and, hence, the electron beam.
Although separate vacuum chambers may be used to house a reticle and a wafer, an entire lens system may generally be housed in a single vacuum chamber. FIG. 1b is a diagrammatic representation of a standard lens system of an electron beam projection system, which is contained within a vacuum chamber. Like lens system 10 of FIG. 1a, a lens system 50 includes an illumination lens 12xe2x80x2 and a projection lens 18xe2x80x2. An electron beam is arranged to pass through illumination lens 12xe2x80x2. The electron beam then passes from illumination lens 12xe2x80x2 to a reticle 16xe2x80x2 which masks out part of the electron beam. After passing through reticle 16xe2x80x2, portions of the electron beam pass through projection lens 18xe2x80x2 and onto wafer 22xe2x80x2. As shown, illumination lens 12xe2x80x2, reticle 16xe2x80x2, projection lens 18xe2x80x2, and wafer 22xe2x80x2 are all contained within a vacuum chamber 60.
A characteristic of electron beam projection systems is the ability to dynamically move a projection image to follow a stage, which is generally not possible with an optical system, as will be appreciated by those skilled in the art. In addition, electron beam lens systems typically correct for relatively small errors in relative stage positions, whereas optical systems generally do not.
It is often desirable to have moving parts associated with an electron beam projection system. For example, a wafer may be placed on a wafer stage which enables the wafer to be positioned beneath a projection column as appropriate. The use of air bearing guides to facilitate the movement of a wafer stage within a vacuum would generally provide for relatively high stiffness, substantially no friction, and low noise while the air bearing guide was moving over a guide beam. However, conventional air bearings typically leak air around their perimeters. Hence, air flow leaks into a vacuum chamber from an air bearing guide would reduce the vacuum level in a vacuum chamber. In general, the allowable leakage flow from an air bearing depends upon the acceptable vacuum level in a vacuum chamber, as well as the vacuum pumping capability associated with the vacuum chamber. Typically, for an electron beam projection system, desired vacuum levels are relatively high, e.g., on the order of approximately 1e-6 Torr. A conventional air bearing generally leaks flow many orders of magnitude above a tolerable, or acceptable, level for an electron beam projection system.
Therefore, what is needed is a method and an apparatus for enabling wafers to be positioned efficiently within an electron beam projection system. That is, what is desired is an air bearing linear guide which is suitable for use in a relatively high vacuum environment.
The present invention relates to air bearings which are suitable for use in an environment with high vacuum levels. According to one aspect of the present invention, an air bearing linear guide that is suitable for use in a vacuum environment is arranged to substantially wrap around a portion of a guide beam without covering the ends of the guide beam. The air bearing linear guide includes a sleeve that has an inner surface, and a first air pad that is located on the inner surface of the sleeve. A first land is arranged on the inner surface of the sleeve at least partially around the first air pad, e.g., such that the land is substantially offset from the perimeter of the air pad. The first land and the air pad define a first channel therebetween which is arranged to be vented to a non-vacuum environment to allow the air pad to function as if it were in a non-vacuum environment, e.g., an atmospheric pressure environment. Finally, the air bearing linear guide includes a second land that is arranged to substantially seal the inner surface of the sleeve from the vacuum environment. Sealing the inner surface of the sleeve from the vacuum environment, e.g., by minimizing the distance between the edge of the sleeve and the guide beam, prevents significant leakage of flow from the air bearing linear guide into a vacuum environment. In one embodiment, the second land is arranged to substantially seal the sleeve against the guide beam.
In another embodiment, a third land is spaced apart from the first land to at least partially define a second channel which is arranged to contain gas at a relatively low vacuum. In such an embodiment, a first area on the inner surface of the sleeve, the first area may also be in communication with the second channel and is, further, arranged to-be in fluid communication with the guide beam.
According to another aspect of the present invention, a guide bearing, the guide bearing which is arranged to interface with a guide beam and is also arranged to move with respect to the guide beam includes a sleeve, a plurality of air pads, a plurality of raised areas, and first and second pluralities of lands. The sleeve has both an outer surface which is exposed to a vacuum and an inner surface. The air pads are included on the inner surface, and are of a first height. Each raised area is arranged to define an individual channel between each raised area and a corresponding air pad, and has a height which is approximately the same as the first height. Each individual channel is arranged to vent to an atmospheric pressure environment. The lands are each arranged to define a channels in cooperation with raised areas and other lands. Channels are defined between each land of the first plurality of lands and raised areas to accommodate relatively low vacuum flow, while channels are defined between each land of the second plurality of lands and each land of the first plurality of lands, to accommodate relatively high vacuum flow.
According to still another aspect of the present invention, a guide bearing system includes an air bearing linear guide and a first guide beam. The air bearing linear guide includes a sleeve with an inner surface on which an air pad is mounted. The air pad is separated from a first land by a first area which substantially surrounds a perimeter of the air pad. The first area is arranged to be in communication with a source of substantially atmospheric pressure. The air bearing linear guide is generally arranged to substantially wrap around a non-end section of the first guide beam and to slide with respect to the guide beam. The guide beam includes a first duct and a second duct which are each in fluid communication with the air bearing linear guide. In one embodiment, wherein a gap is defined between a top surface of the air pad and an outer surface of the first guide beam that is approximately equivalent to a flying height of the air bearing linear guide.
In one embodiment, the first land is separated from a second land by a second area, and the second area being arranged to be in fluid communication with the first duct. In such an embodiment, the second land may be separated from a third land by a third area which is in fluid communication with the second duct. The guide bearing system may also include a second guide beam which includes a first section and a second section. The second guide beam maybe coupled to the air bearing linear guide such that a first transfer port of the air bearing linear guide is in fluid communication with the second area and the first section, and a second transfer port of the air bearing linear guide in fluid communication with the third area and the second section.
These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings.